Demonstration of serotoninergic axons terminating on luteinizing hormone-releasing hormone neurons in the preoptic area of the rat using a combination of immunocytochemistry and high resolution autoradiography.

The synaptic relationship between serotoninergic terminals and luteinizing hormone-releasing hormone-containing neurons was investigated in the medial preoptic area using a combined technique. Axon terminals selectively taking up 5-[3H]hydroxytryptamine were labelled autoradiographically and luteinizing hormone-releasing hormone-containing neuronal elements were identified by means of immunocytochemistry. Synaptic contacts were observed between tritiated 5-hydroxytryptamine-labelled boutons and luteinizing hormone-releasing hormone-immunoreactive dendrites. About 5% of the boutons which formed synapses with luteinizing hormone-releasing hormone-immunoreactive dendrites were found to be labelled by the tritiated indolamine. Luteinizing hormone-releasing hormone-immunoreactive axon terminals occurred as presynaptic elements in contact with unidentified dendritic spines, shafts or perikarya. These observations provide morphological basis for the idea that 5-hydroxytryptamine-containing neurons can act directly on luteinizing hormone-releasing hormone release. Further, they support the assumption that luteinizing hormone-releasing hormone is not only a neurohormone but may also function as a neurotransmitter or neuromodulator.